Create a blank single page

Hi,

I would like to create a single page and remove everything (header and footer) else in the page.
I tried the Custom CSS with this code

.page-id-208738 .site-header,
.page-id-208738 .site-footer {
display: none;
}

But it does not work. How can we remove everything (menu, logo, etc) for a single page?
Because I would like to brand it in another way.
Thank you

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